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I have two unused USB 2 drives. I wanted to stripe them in a RAID0 configuration for fast disk access for virtual machines. (I find running a VMware virtual machine off a USB2 drive to be painfully slow. Especially Windows Vista)

If I have both USB drives attached to the same USB2 hub, will that negate any benefit I gain by creating a RAID0 array? That is to say, is the speed of USB2 the limiting factor or is the speed of the drives? Would I get better performance by attaching one or both drives directly to my computer?

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Better tag suggestions welcomed! – Josh Mar 15 '10 at 17:29
up vote 1 down vote accepted

The throughput on USB 2.0 is 480Mbps. That is both ways so you can achieve about 240Mbps writing/reading. If you want to take full advantage of RAID be sure to use SATA 3Gbps or SCSI.

What happens is lets say for example you send a 480MB file to the RAID0 array through USB it is cut in half (at least) and the rest is cached so that more data can be sent; the USB Hub processes the data to the HDD's small pieces at a time (<=240Mbps) whereas SATA can process it at <= HALF of SATA 3.0Gbps for example.

This is one reason why you see External HDD's sold with 5400RPM and not 7200RPM (because it's overkill; you will never use the additional 1800RPM because USB is not fast enough yet).

To let someone who may not know. When I say half I am referring to broadband communication. This is because all (modern) media takes into account reading and writing. So you can read at most half the speed of the said speed. Another example is 802.11g 54Mbps TOTAL read AND write... Cut that in half because it's a broadband media and you can at most read and/or write at 27Mbps.

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Um. The 240 Mbps (480÷2) is something you invented out of thin air. I have managed (with a LaCie enclosure, having both USB 2 HiSpeed and FireWire 400 connectivity) to get a 32 MiB/s throughput using USB (and 36 MiB/s using FireWire); the enclosure contains a Samsung disk, model MP0804H . This comes close to the observed maximum as described in the Wikipedia page. – tzot Apr 21 '10 at 2:03
@ΤΖΩΤΖΙΟΥ Yes you can get up to 32MBps, however attempt to copy from the drive; it will be cut in half to accommodate the additional over-head. Ethernet, WiFi, among every other BROADBAND connection do this. – anon31097 Apr 26 '10 at 17:23
@m00st: Yes, WiFi does it. However, Ethernet hasn't done this since its 10 Mbit/s half-duplex days. And I don't understand your “attempt to copy from the drive”; the 32MBps I achieved was when I was doing a dd of the USB drive to a file in the main SATA drive, so I was copying from the drive. Still your “can achieve about 240 MBps writing/reading” is incorrect. – tzot May 3 '10 at 15:53
@ΤΖΩΤΖΙΟΥ: It does this for dual band connections; fact. Take 480Mbps and divide that by 8. 480 Mega BITS per second is 60MBps. You've confirmed with me the mere fact that it's cut in half. 480Mbps != MBps. – anon31097 May 3 '10 at 16:12
@m00st: Let's start with the basics: in my previous comment, I made a typo in copying your statement and wrote 240 MBps while I meant 240 Mbps. Agreed? Good. Now, 240 Mbps = 30 MBps < 32 MBps which I have achieved, ergo your “That is both ways so you can achieve about 240 Mbps writing reading” is a piece of misinformation. Agreed? You won't agree, I know, so go ask a friend of yours who knows about USB and show him your answer and the comments. They'll hopefully explain. – tzot May 7 '10 at 13:58

I would say the only definitive way is to try it and benchmark before and after.

My best hypothesis - for short bursts (small reads and what not), you might see a performance improvement. For sustained transfers, most likely not.

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While I appreciate the technical nature of the earlier answer, I don't know if it addressed the OP question of the hub effect. My understanding of USB devices is that (unlike ethernet) the only communication is from device to computer, so a hub would seem to slow you down in a RAID-like situation. Having said that, there is no guarantee that what appears to be "individual" USB ports on your computer might be "hubbed" inside the case...

P.S. A newer/faster USB 3 standard is emerging in 2011, but the frustration RAID-seekers encounter is that many new machines supporting USB 3 only do so on ONE port! And there are precious few USB 3 hubs...

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